1. Field of the Invention
This invention relates to a radiation image erasing apparatus. This invention particularly relates to a radiation image erasing apparatus, wherein fluorescent lamps are employed as a source for producing erasing light.
2. Description of the Prior Art
Techniques for reading out a recorded radiation image in order to obtain an image signal, carrying out appropriate image processing on the image signal, and then reproducing a visible image by use of the processed image signal have heretofore been known in various fields. Also, it has been proposed to use stimulable phosphors in radiation image recording and reproducing systems. Specifically, a radiation image of an object, such as a human body, is recorded on a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet). The stimulable phosphor sheet, on which the radiation image has been stored, is then exposed to stimulating rays, such as a laser beam, which cause it to emit light in proportion to the amount of energy stored thereon during its exposure to the radiation. The light emitted by the stimulable phosphor sheet, upon stimulation thereof, is photoelectrically detected and converted into an electric image signal. The image signal is then processed and used for the reproduction of the radiation image of the object as a visible image on a recording material.
In the radiation image recording and reproducing systems, wherein the stimulable phosphor sheets are used, after the radiation image has been read out from a stimulable phosphor sheet, the stimulable phosphor sheet is exposed to erasing light, which is produced by a fluorescent lamp, or the like, and energy remaining on the stimulable phosphor sheet is thereby released. The erased stimulable phosphor sheet can again be used for the recording of a radiation image. Therefore, the erasing step for irradiating the erasing light to the stimulable phosphor sheet must be carried out after the step for reading out the radiation image from the stimulable phosphor sheet. A radiation image erasing apparatus for carrying out the erasing operation is often incorporated as a portion of a radiation image read-out apparatus. Ordinarily, straight-tube fluorescent lamps are utilized as an erasing light source. Also, as in an example of a conventional radiation image erasing apparatus 20 shown in FIG. 6A, a stimulable phosphor sheet 11 is exposed to the erasing light, which is produced by straight-tube fluorescent lamps 21, 21, . . . , while the stimulable phosphor sheet 11 is being conveyed in a direction that is normal to the direction (the longitudinal direction) along which each of the straight-tube fluorescent lamps 21, 21, . . . extends. In this manner, the entire area of the stimulable phosphor sheet 11 is erased.
In the erasing step, it is necessary for erasing energy of a level at least equal to a predetermined threshold value ETh to be given to the entire area of the stimulable phosphor sheet 11. If the level of erasing energy given to a portion of the stimulable phosphor sheet 11 is lower than the threshold value ETh, non-uniformity in level of erasing of the stimulable phosphor sheet 11 will occur, and adverse effects will occur on the image signal detected from the stimulable phosphor sheet 11, on which the next radiation image has been stored.
With the straight-tube fluorescent lamps 21, 21, . . . , which are employed in the example of the conventional radiation image erasing apparatus 20 shown in FIG. 6A, the distribution of the illuminance in the direction, along which each of the straight-tube fluorescent lamps 21, 21, . . . extends, takes a pattern as shown in FIG. 6B.
Specifically, the illuminance is highest (i.e., equal to Dmax) at a middle portion of each of the straight-tube fluorescent lamps 21, 21, . . . and is low (i.e., equal to D.sub.0) at portions in the vicinity of electrodes 22a and 22b, which are located at the two ends of each of the straight-tube fluorescent lamps 21, 21, . . . The level of the illuminance represents the level of erasing energy. Therefore, it is necessary for the irradiating conditions (i.e., the irradiation time, the illuminance, and the like) of the straight-tube fluorescent lamps 21, 21, . . . to be set such that the illuminance at the portions in the vicinity of the electrodes 22a and 22b, which takes the lowest illuminance value D.sub.0, may be higher than a threshold value DTh in the illuminance region corresponding to the aforesaid threshold value ETh of erasing energy. However, in such cases, at the middle portion of each of the straight-tube fluorescent lamps 21, 21, . . . , at which the illuminance is originally high (i.e., as high as Dmax),the level of erasing energy becomes excessively high, and useless high erasing energy is imparted to the stimulable phosphor sheet 11.
Also, in an example of the conventional radiation image erasing apparatus 20 shown in FIG. 7A, which may be incorporated as a portion of a radiation image read-out apparatus, a straight-tube fluorescent lamp 21 is employed as the erasing light source. In this example, the temperature distribution and the illuminance distribution in the longitudinal direction of the straight-tube fluorescent lamp 21 take patterns shown in FIG. 7B and FIG. 7C.
As is understood from FIG. 7B, the temperatures at portions other than the middle portion of the straight-tube fluorescent lamp 21 are higher than the temperature at the middle portion. Also, as illustrated in FIG. 7C, the illuminance distribution is affected by the temperature distribution shown in FIG. 7B.